J. N. Pinkerton, Research Plant Pathologist, Horticultural Crops Research Laboratory, USDA, ARS, Corvallis, OR 97330; M. L. Canfield, Senior Faculty Research Assistant, K. L. Ivors, Faculty Research Assistant, and L. W. Moore, Professor, Dept. of Botany and Plant Pathology, Oregon State University, Corvallis, OR 97331.

The nursery industry has a major economic impact in the Pacific Northwest, with annual revenues in excess of $400 million. Soilborne pathogens and pests cause substantial losses in many perennial, herbaceous, and woody nursery crops in the region. Hundreds of thousands of dollars of nursery crops are estimated to be lost on an annual basis due to Phytophthora species. Verticillium dahliae is also an important pathogen, particularly on maple. Crown gall, caused by pathogenic Agrobacterium species, is an important disease with estimated annual losses as high as $400,000. Interstate movement of nursery stock infected with plant parasitic nematodes such as Pratylenchus penetrans may be restricted, causing an economic loss to nurseries. With the impending loss of many chemicals such as methyl bromide, greater losses from these pathogens may be expected.

During the last 2 years, soil solarization has been evaluated alone and in combination with cover cropping or applications of metam sodium as alternatives to methyl bromide for controlling these pathogens. Replicated field plots were established in a silty clay loam soil near Corvallis, OR. The main treatments were solarized and nonsolarized plots and subplots consisted of (a) green manures from cover crop, (b) application of metam sodium, (c) clean fallow plots, and (d) noninoculated control plots.

Peppermint plants infested with P. penetrans were planted in the plots in April 1994 to establish nematode populations. On May 22, 1995, V. dahliae and P. cinnamomi inocula were broadcast on the soil surface and the plots were rotovated to a depth of 20 cm. The cover crops, 'Trudan 8' sudangrass, 'Mica' barley, and 'Dwarf Essex' rape, were planted in one-half of the plots. The remaining plots were maintained as clean fallow controls or treated with metam sodium. On July 22, the cover crops were chopped, a suspension of A. tumefaciens was sprayed on the soil surface, annual bluegrass seed dispersed on the plots and they were rotovated to 20 cm. Plots were then irrigated to thoroughly wet the soil. Metam sodium plots were sprayed with 235 or 935 L ha-1 (the recommended rate), rotovated to 20 cm, and rolled to seal the soil surface. Finally, a 0.6-mil plastic film was then stretched over solarized plots. The plots were solarized for 2 months from July 22 to September 19. Soil temperatures were monitored in solarized and nonsolarized soils at 5, 10, and 20 cm. The summer of 1995 was slightly cooler than normal. However , the mean and maximum daily soil temperatures in solarized plots at all depths were 30 to 38 ° C and 40 to 50 ° C, respectively, 8 to 10 ° C higher than nonsolarized soil.

To assay population densities of the pathogens, soil samples were collected in May when cover crops were planted, in July at the start of solarization, 1 month later in August, and when the tarps were removed in September. In addition, nylon bags containing soil inoculated with V. dahliae and P. cinnamomi were buried in each plot at depths of 5, 10, and 20 cm in July. Pathogen population densities in the soil bags were assayed in August and September. The effect of treatments on weed populations was evaluated by quantifying the emergence of seedlings in the plots and in plot soil potted in the greenhouse. In general, soil solarization greatly reduced the population densities of V. dahliae, P. cinnamomi, A. tumefaciens, and P. penetrans. However, solarization, cover cropping, and cover cropping followed by solarization were not as effective as metam sodium at the recommended rate. Population densities of the pathogens were not significantly different in soil collected after 30 or 60 days of solarization. The effective depth of solarization was dependent on the pathogen. P. cinnamomi was rarely recovered in soil buried at 20 cm in solarization-cover crop treatments, but V. dahliae inoculum was not significantly reduced at this depth. The emergence of annual bluegrass and other weeds was significantly reduced in solarized soil. In June 1996, susceptible woody host plants were planted in the plots. These plants will be evaluated for disease incidence and severity for 1 year.

Depending on the organism, cover cropping was neutral in effect or actually increased pathogen populations. Since agrobacteria survive better on plant material, the cover crops may have enhanced the survival of agrobacteria in nonsolarized soil and gave no added benefit to solarization. Solarization and cover cropping did not reduce population counts of fluorescent pseudomonads, spore-forming bacteria, and Actinomycetes, all potentially beneficial microorganisms. Our results indicate that soil solarization has the potential for nonchemical management of important soilborne pathogens in this region of the U.S. Over the next year we will determine how solarization and its impact on the pathogen populations relate to disease expression in selected perennial hosts. Further investigations are needed to determine the conditions in which solarization may be an effective and practical method for control of soilborne diseases of perennial plants.